Hydrocarbons and Alcohols

Hydrocarbons contain only carbon and hydrogen atoms — the simplest organic molecules, and the foundation for more complex compounds. Alcohols are one step more complex: they contain at least one hydroxyl (-OH) group, which significantly increases their reactivity. Building on the general IUPAC naming framework, this article covers how to name alkanes, haloalkanes, alkenes, alkynes, alcohols, and diols.

Key Takeaways

• Alkanes are saturated hydrocarbons (CₙH₂ₙ₊₂, all single bonds), named with the -ane suffix.

• Haloalkanes name halogen substituents with fluoro-/chloro-/bromo-/iodo- prefixes, alphabetized with other substituents (e.g., 2-bromo-2-methylpentane).

• Alkenes (-ene) and alkynes (-yne) are numbered so the double or triple bond gets the lowest possible locant.

• Alcohols (-ol, replacing the parent's -e ending) outrank alkenes, alkynes, and alkanes for numbering priority, due to the hydroxyl carbon's higher oxidation state. When an even higher-priority group is present, the hydroxyl becomes a hydroxy- prefix instead.

• Diols have two hydroxyl groups: geminal (same carbon, e.g. pentane-3,3-diol) or vicinal (adjacent carbons, e.g. ethane-1,2-diol) — named by adding -diol to the full parent hydrocarbon name.

• Worked example: pent-4-en-2-ol shows the hydroxyl group controlling the numbering over a competing double bond.

Alkanes — Saturated Hydrocarbons

Alkanes are hydrocarbons with the general formula CₙH₂ₙ₊₂. They're considered saturated because every carbon-carbon bond is a single bond — the -ane suffix signals this: if a name ends in -ane, the structure contains only single bonds.

The parent-chain names for alkanes (methane through decane) follow the same pattern established for naming any organic molecule — see the complete reference table in "IUPAC Naming Conventions."

Haloalkanes — Naming Halogen Substituents

Halogens — fluorine, chlorine, bromine, and iodine — are common substituents on hydrocarbon chains. They're named with the prefixes fluoro-, chloro-, bromo-, and iodo-, and alphabetized alongside any other substituents just like any other prefix.

Worked example: a bromine atom attached to a five-carbon chain, alongside a methyl group on the same carbon, is named 2-bromo-2-methylpentane. The parent chain is pentane (5 carbons); "bromo" indicates the bromine substituent, and both the bromo and methyl groups sit on carbon 2. Alphabetically, "bromo" comes before "methyl," so bromo is listed first.

Alkenes and Alkynes — Naming Double and Triple Bonds

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. To name one, replace the parent chain's -ane suffix with -ene, and number the chain so the double bond gets the lowest possible locant. For example, a double bond between carbons 2 and 3 is named using the number 2 — the locant marks where the double bond starts.

Alkynes work the same way for triple bonds: the suffix -yne replaces -ane, and the chain is numbered to give the triple bond the lowest possible locant. A triple bond between carbons 1 and 2 is numbered starting at carbon 1.

Alcohols — Naming the Hydroxyl Group

Alcohols are hydrocarbons containing a hydroxyl (-OH) group. To name one, replace the parent hydrocarbon's -e ending with the suffix -ol. The simplest alcohol is ethanol — a two-carbon chain (ethane) with one hydrogen replaced by a hydroxyl group.

In more complex molecules, the chain is numbered to give the hydroxyl group the lowest possible locant, as in 5-methyl-2-heptanol, where the hydroxyl sits on carbon 2 and a methyl substituent sits on carbon 5.

Why the hydroxyl group wins numbering priority: alcohols outrank double bonds, triple bonds, and plain alkanes in the functional-group seniority order established in "IUPAC Naming Conventions." This is because of the hydroxyl carbon's higher oxidation state — in organic chemistry, oxidation generally means more bonds to oxygen (or other electronegative atoms) and fewer bonds to hydrogen. So when numbering the chain, the hydroxyl group gets the lowest locant even if that pushes other substituents or bonds to higher numbers.

If an even higher-priority functional group is present — a ketone, for instance — the hydroxyl no longer gets the -ol suffix. Instead, it's named as a substituent using the prefix hydroxy-.

Diols — Geminal vs. Vicinal

A diol is a molecule with two hydroxyl groups. How they're arranged determines the term used:

• Geminal — both hydroxyl groups are on the same carbon. Example: pentane-3,3-diol, where both hydroxyls sit on carbon 3.

• Vicinal — the hydroxyl groups are on adjacent carbons. Example: ethane-1,2-diol, with hydroxyls on carbons 1 and 2.

To name a diol, keep the entire parent hydrocarbon name intact and add the suffix -diol, numbering each hydroxyl group's position.

Worked Example: Naming a Molecule with Both an Alkene and an Alcohol

Consider a five-carbon chain with a hydroxyl group and a carbon-carbon double bond between the last two carbons.

• Parent chain: 5 carbons → pentane base.

• Priority for numbering: the hydroxyl group outranks the double bond, so the chain is numbered from the end nearest the hydroxyl, not the end nearest the double bond.

• Numbering: with the hydroxyl on the second carbon from one end, that end becomes carbon 1 — placing the hydroxyl at carbon 2 and the double bond between carbons 4 and 5.

• Suffix: the alcohol gets the -ol suffix (it's the higher-priority group present); the double bond is noted with its locant before the -ol suffix.

Complete name: pent-4-en-2-ol

This demonstrates the priority rule directly: even though numbering from the opposite end would give the double bond a lower locant, the hydroxyl group's higher priority controls the numbering instead.

Common MCAT Mistakes

• Numbering a double or triple bond as if it were a substituent-locant problem. Alkenes and alkynes get the lowest possible locant for the multiple bond itself — this isn't the same rule as minimizing a substituent locant set, and mixing the two up leads to the wrong numbering direction.

• Forgetting that alcohols outrank alkenes and alkynes for numbering priority. When both a hydroxyl group and a double or triple bond are present, the hydroxyl always gets the lowest locant, even if that gives the multiple bond a higher number.

• Swapping geminal and vicinal. Geminal diols have both hydroxyls on the same carbon (e.g., pentane-3,3-diol); vicinal diols have them on adjacent carbons (e.g., ethane-1,2-diol) — the terms are easy to reverse under time pressure.

• Using the -ol suffix when a higher-priority group is present. If a ketone, aldehyde, or other group senior to alcohols is in the molecule, the hydroxyl is demoted to a hydroxy- prefix rather than keeping the -ol suffix.

MCAT-Style Concept Check

Question: A six-carbon chain contains one hydroxyl group and one carbon-carbon double bond. Numbering from the left end places the hydroxyl on carbon 2 and the double bond between carbons 4 and 5. Numbering from the right end would instead place the double bond between carbons 2 and 3, with the hydroxyl on carbon 5. Which numbering direction is correct?

• A) Numbering from the right end, because double bonds always take priority over hydroxyl groups

• B) Numbering from the left end, because the hydroxyl group outranks the double bond in the functional-group seniority order, so it gets the lowest locant

• C) Either direction, since the hydroxyl and double bond locants are compared by their sum

• D) Numbering from the right end, because alphabetizing the substituent names determines the numbering direction

Answer: B

Explanation: Alcohols outrank alkenes in the functional-group seniority order, so the hydroxyl group must receive the lowest possible locant — even if that means the double bond ends up with a higher number. Numbering from the left gives the hydroxyl carbon 2 (versus carbon 5 from the right), so the left-end numbering is correct. Option A reverses the actual seniority order. Option C misapplies a rule that only governs comparing sets of same-priority locants (like multiple substituents), not competing functional groups. Option D confuses the alphabetizing rule (used only to order substituent prefixes in the final name) with the numbering-priority rule.

FAQ

What's the difference between an alkene and an alkyne?

An alkene contains at least one carbon-carbon double bond and is named with the -ene suffix; an alkyne contains at least one carbon-carbon triple bond and is named with the -yne suffix. Both are numbered so the multiple bond gets the lowest possible locant.

How do you name halogen substituents on a hydrocarbon chain?

Halogens are named as prefixes — fluoro-, chloro-, bromo-, and iodo- — and alphabetized alongside any other substituents on the chain, just like any other substituent prefix.

Why does a hydroxyl group get numbering priority over a double or triple bond?

Alcohols sit higher than alkenes, alkynes, and alkanes in the functional-group seniority order, largely because of the hydroxyl carbon's higher oxidation state. So when a chain is numbered, the hydroxyl group always receives the lowest possible locant, even if that pushes a double or triple bond to a higher number.

What's the difference between a geminal and a vicinal diol?

A geminal diol has both hydroxyl groups on the same carbon (e.g., pentane-3,3-diol). A vicinal diol has the two hydroxyl groups on adjacent carbons (e.g., ethane-1,2-diol). Both are named by adding the -diol suffix to the full parent hydrocarbon name.